Carbon Dioxide and Methane Emission into the Atmosphere and Its Relationship with Chemogenic Sedimentation in the Hypersaline Lake Baskunchak (Russia)

Baskunchak is a large drainless, highly saline lake located in the Caspian lowland. The chemical and gas composition of water (brine) and bottom sediments lying under a 10 cm layer of salt in the lake has been studied; specific fluxes of CH4 and CO2 at the water–atmosphere interface have been measured. The lake’s sodium chloride brine is characterized by high mineralization (313.5–334.7 g/L) and a slightly acidic–neutral pH (5.75–6.80). Bottom sediments are characterized by a slightly acid–neutral pH (6.27–6.64) and a reducing condition (Eh from −104.7 to +22.0 mV). Specific fluxes of CH4 into the atmosphere were low (0.11–0.12 mg CH4/(m2 h)) due to its low concentrations in the brine of the lake (0.91–2.66 µL/L). The appearance of an excess of HCO3 during the anaerobic oxidation of CH4 in the bottom sediments of the lake contributes to the formation of autigenic gypsum and calcite. Specific CO2 fluxes into the atmosphere ranged from 12.2 to 73.1 mg CO2/(m2 h). The probable source of CO2 in the brine of the lake and its emission into the atmosphere, in addition to the process of organic matter cycling and uptake by microorganisms, is the chemogenic precipitation of sulfates and calcium carbonates.